Perhaps I will get downvoted to death again for saying so, but the obvious answer is because the name "rationalist" is structurally indistinguishable from the name "scientology" or "the illuminati". You attract people who are desperate for an authority to appeal to, but for whatever reason are no longer affiliated with the church of their youth. Even a rationalist movement which held nothing as dogma would attract pe…
And I say this as a Christian. I often think that becoming a state religion was the worst thing that ever happened to Christianity, or any religion, because then it unavoidably becomes a tool for power and authority.
And doing the same with other ideas or ideologies is no different. Look at what happened to communism, capitalism, or almost any other secular idea you can think of: the moment it becomes established, accepted, and official, the corruption sets in.